Suddenly, everyone agreed, catching cold (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'everyone agreed catching cold' is the wordplay.
'everyone' becomes 'all' (all people).
'agreed' becomes 'at one' (to be 'at one' is to be agreed).
'catching' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'atone' going around 'c' is 'atonce'.
'all'+'atonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "in unison" , "Together" , "Simultaneously - immediately" , "How daffodils appeared to Wordsworth" , "Without warning" .)